As an example of the tire air pressure detecting apparatus, a direct-type tire air pressure detecting apparatus has been conventionally known. In the direct-type tire air pressure detecting apparatus, a transmitter is directly fixed to each of wheels with a tire. The transmitter is provided with a sensor, such as a pressure sensor. A vehicle body is equipped with an antenna and a receiver. When the transmitter transmits a detection signal of the sensor, the receiver receives the detection signal through the antenna, and the tire air pressure of the wheel is detected based on the detection signal.
In such a direct-type tire air pressure detecting apparatus, data transmitted from the transmitter includes individual ID information for identifying whether the transmitted data is associated with a subject vehicle to which the tire air pressure detecting apparatus is equipped, and for identifying the wheel to which the transmitter transmitting the data is fixed.
To specify the position of the transmitter based on the ID information of the transmitted data, the receiver needs to store beforehand a relationship between the ID information of each transmitter and the position of each wheel. When the positions of the wheels are changed by a tire rotation, it is necessary to register the relationship between the ID information of each transmitter and the position of each wheel again. For example, a patent literature 1 proposes a wheel position detecting device that is capable of automatically registering the relationship between the ID information of each transmitter and the position of each wheel.
The wheel position detecting device of the patent literature 1 performs wheel position detection to detect, using the detection signal of the wheel speed sensor, the position of the wheel to which the transmitter is equipped in the vehicle.
Specifically, a wheel speed sensor detects the number of teeth of a gear that pass besides the wheel speed sensor with rotation of the wheel, and gear information indicating a tooth position of the gear is obtained based on a detection signal of the wheel speed sensor. Next, a variation allowance range is set based on the tooth position at a reception timing of a frame. When the tooth position at the reception timing of the frame is not inside of the variation allowance range, the wheel corresponding to the gear is excluded from candidate wheels to which the transmitter that has transmitted the frame. Then, a remaining wheel is registered as the wheel to which the transmitter that has transmitted the frame. Moreover, an overlapping range between the variation allowance range based on the tooth position at the reception timing of the frame and the variation allowance range set based on the tooth position of the great at a previous reception timing of the frame is set as a new variation allowance range. In this way, a new variation allowance range is narrowed.
As described above, since the frame is transmitted when the transmitter is at a predetermined angular position of the tire, the tooth position at the reception timing of the frame is learned to perform the wheel position detection. Therefore, the wheel position can be further promptly and accurately detected.